Evaluation of the cone-shaped pickup performance for low charge sub-10 fs arrival-time measurements at free electron laser facilities

Abstract

An evaluation of the cone-shaped pickup performance as a part of the high bandwidth bunch arrival-time monitors (BAMs) for a low charge sub-10 fs arrival-time measurements is presented. Three sets of pickups are installed at the free electron laser FLASH at Deutsches Elektronen-Synchrotron, the quasi-cw SRF accelerator ELBE at the Helmholtz-Zentrum Dresden-Rossendorf and the SwissFEL injector test facility at Paul Scherrer Institute. Measurements and simulations are in good agreement and the pickups fulfill the design specifications. Utilizing the high bandwidth BAM with the cone-shaped pickups, an improvement of the signal slope by a factor of 10 is demonstrated at ELBE compared to the BAM with a low bandwidth.

Pickup concepts for ultra-low charged short bunches in X-Ray Free-Electron Lasers

“…The RF unit so far comprises four identical pickups mounted circularly around the beam line. The combination of opposite pickup signals compensates for the orbit dependency [16]. The original pickups used at FLASH were of button-type and designed for a 10 GHz bandwidth [17].…”

“…The proportionality to the bunch charge was experimentally proven for the signal slope of the cone shaped pickups at FLASH [16]. The inverse proportionality to r p has to be analyzed in simulations, since these measurements are expensive and limited by the facilities design parameters.…”

“…These are a super relativistic (v = c 0 ) Gaussian bunch with σ z = 1 mm and a charge of 20 pC. Therefore, the voltage can be compared to the well-described state-of-the-art pickups according to Angelovski et al [13,16].…”

“…The increased bandwidth TABLE I. Specifications of the 90 GHz-pickup [28], the modified pickup (2 nd Gen.) [16] and the original (1 st Gen.) [12] a Not specified in the publication leads to a reduced rise time of 7.9 ps but cannot compensate for the missing protrusion and the smaller active area. Thus, the pickup cannot outperform the 1 st generation, but a smaller peak-to-peak voltage gives advantages in machine protection.…”

SwissFEL: The Swiss X-ray Free Electron Laser

“…The bunch arrival-time monitors (BAMs) [83] developed for SwissFEL provide non-destructive, shot-to-shot arrival time information relative to a highly stable pulsed optical reference (see Section 5.1) with resolution better than 5 fs and less than 10 fs drift per day [84,85]. The electron beam generates an S-shaped bipolar transient with a steep slope (15 ps peak-to-peak) in a pick-up with 40 GHz bandwidth [86]. This pickup-signal is probed by a single reference laser pulse and the arrival time is encoded in its amplitude.…”